The directional filtering of the outer ears is a significant step during the evaluation of spatial information. A computer-controlled dummy-head measurement system was developed to measure the HRTFs in huge amount and with increased spatial resolution. Effects and common properties of objects near the head (glasses, hair, baseball cap) were measured. Changes in the HRTFs due to the acoustical environment will be presented as function of frequency and azimuth for different elevations on 2D polar diagrams. Differences of about 1 dB are evaluated in the fine structure of the measured HRTFs and conclusions are drawn for the relevance of HRTF reproduction in binaural synthesis.

A simple spectral computation method is introduced for the evaluation of differences in dummy-head Head-Related Transfer Functions (HRTFs). These transfer functions describe the directional-dependent filtering of the outer ears and are responsible for basic localization cues. Head-Related Transfer Function Differences (HRTFDs) are defined as the quotient of measured HRTFs in the same direction, comparing the cases of modified environmental conditions. The HRTFDs are relative measures and thus free from systematic errors of the measurement system and individual parameters. Our experience shows that HRTFDs are well suited to analyze the fine structure of measured HRTF data and for representation of the changes in the acoustical environment near or on the head. HRTFDs of a BK dummy-head will be shown representing differences from about 1 dB. This measurement accuracy was determined by replacing the dummy-head with an omnidirectional microphone.